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Old 11th December 2003, 03:38 PM   #1
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Default High Power P3A design questions.

Hi, I have been looking at building myself a version of the P3A amp from Rod's pages, but I have a few questions about the design and it's implementation.

Firstly, Rod states that teh maximum rail voltage which should be used with this design should be +/- 40V to get about 100W into an eight ohm load. However, as I will be building this with the newer onsemi output devices and drivers which are rated at up to 230V (I think, that is just a figure from memory, it could be 350 but I don't think it is any lower than 200V) as well as doubling the number of output devices, would it be safe for me to run this design on higher rails? As I was considering going as high as +/- 50V to get around 125W out of each channel.

The second point is concerning the aditional output devices. Now, in the article, Rod states that the high frequency rolloff of the amp is coverned by a descreet capacitor added to aid stability. So, would I be correct in thinking that the extra juction cappacitance seen by the drivers from the extra output transistors will not have to great an effect on the amps frequency responce as this is not the limiting factor governing the circuit design?

Anyway, I have included my updated scheematic below, along with a provisiona copy of the PCB layout and would apreciate comments on both, as well as teh questions posted above.

Andrew.
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Old 11th December 2003, 03:44 PM   #2
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Here is my provisional PCB layout to with the above design. I intend on testing this for a single channel on a single sided PCB, but if it goes well, I intend to make the board double sided and get few professionaly made.

Also, if anyone knows a good scource of low value, high power resistors that can be end mounted in the Uk can you pleasse post here, as I am having trouble finding any at the moment.

andrew.
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Old 11th December 2003, 03:57 PM   #3
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Quote:
Originally posted by bigparsnip
Here is my provisional PCB layout to with the above design.

andrew.

andrew, that's one pretty design job. If you don't mind, can I ask what software package you used for it?

I am running protel dxp demo and had a hard time to find out how to "enlarge" the traces and kind of make it fill-out empty spaces like you did.

Any tip would be appreciated.
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Old 11th December 2003, 04:03 PM   #4
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I did that one with the trial version of eagle ( http://www.cadsoftusa.com/), which is a joy to use, but is unfortunately rather limited with some of the design work you can do (only two layers, and max PCB size of 100 X 80 mm). So I also 'upgraded' recently to a coppy of protell 99SE, but I am also finding the lerning curve a little steep (I have just about managed work out how to transfer a schematic to PCB drwing now, but it seems to take a while to set up if you don't have any of your own libraries to work form). As far as the fills go in protell, I'm afriad I can't help you with that yet, but I should get round to having a bit more explore with it soon, and if I find out how I'll let you know.
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Old 11th December 2003, 04:06 PM   #5
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Here is another coppy of the board, but without the component overlay (which isn't finished yet anyway) so you can get a better look at the track layout.
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Old 11th December 2003, 04:27 PM   #6
sobazz is offline sobazz  Denmark
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Pay attention to your driver transistors. You may need to replace those with a higher rated type.
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Old 11th December 2003, 04:45 PM   #7
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Hi, the driver transistors will also be the new, very well specified, models MJE15034 and MJE15035 from onsemi, both being rated at 350V (it was 350 for the outout devices too, 230W for their power handling) and up to 4A each.

What about the small signal transistors that are on there? Is there anything better thant he BC546 to use, as I always thought that these were a cheep generic component, so there must be something that would be better out there to replace them?
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Old 11th December 2003, 05:02 PM   #8
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I think Rod told me to use the mpsa42 and mpsa92's for high power p3a's....someone might need to confirm that though, I can't remember. I'm pretty sure that they'll work.

-Matthew K. Olson
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Old 11th December 2003, 05:07 PM   #9
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Hmm, not the fastest devices in the world (only 50MHz, compared to the 35MHz for the output devices seems a little slow) but I guess they would more than do their job properly and the specs for current and voltage handling seem ok. I guess I could always give a few different types a go and see if any are noticably better in there.
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Old 11th December 2003, 05:33 PM   #10
Freddie is offline Freddie  Sweden
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I use 2N5551 small signal transistors, in my P3A running at +/-52V.

http://www.fairchildsemi.com/ds/2N/2N5551.pdf

/Freddie
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